Optical channel monitor with integral optical switch

ABSTRACT

A multiport optical switch (such as an N×1 switch) is used to controllably select a specific incoming optical signal that is to be processed by an associated optical channel monitor (OCM). The OCM includes a tunable optical filter and photodetector arrangement, and is configured to measure the optical spectrum of the incoming optical signal and extract information associated with the various optical channels (wavelengths) forming the incoming optical signal (i.e., power, wavelength, OSNR and the like for each channel). The OCM also includes a signal processing component that generates a pair of output control signals, a first signal to control the wavelength scanning process of the tunable optical filter and a second signal to control the setting of the multiport optical switch.

TECHNICAL FIELD

The present invention relates to optical channel monitors (OCMs) and,more particularly, to an OCM that incorporates optical switchingfunctionality for use in multiport optical monitoring applications.

BACKGROUND

Optical networks are found in a wide variety of high speed applications,and used to provide efficient transmission of voice, video and datasignals. Some optical networks implement wavelength divisionmultiplexing (WDM) to increase network bandwidth. In WDM opticalnetworks, multiple optical channels occupying distinctwavelengths/frequencies are multiplexed into a single optical signal fortransmission through a single optical fiber.

Error rates in long-haul WDM optical networks depend on, among otherthings, per channel optical power and optical signal-to-noise ratio(OSNR) values. Modules such as optical amplifiers have been found todegrade the OSNR, as well as produce power ripple across the opticalband for the transmission channels. To remedy these problems, WDMoptical networks often implement systems that perform optical channelpower monitoring and/or optical channel power correction to maintainoptimal channel powers and desired low error rates.

Various types of optical channel monitors (OCMs) have been developed toperform these functions, and in general are configured to measure themultiple wavelengths used within a WDM network, with OCMs disposed atvarious locations throughout the network. The power level of eachoptical channel may be reported in real time, with feedback from the OCMto a “host” (such as a Network Management System (NMS)) utilized tooptimize the optical power level for each channel, identify performancedrift, and verify system functionality.

More complex networks may use dense WDM (DWDM) systems, which supportthe communication of a large number of separate optical fibers (ports),each port supporting multiple optical channels. Monitoring equipment forthese DWDM systems becomes increasingly expensive and time-consuming touse, requiring multiple measurements of each signal as they pass throughmany optical elements in the system. Additionally, one or more of thesignals appearing at a monitoring arrangement may be of relatively lowpower, making it difficult to accurately measure the signal level in thepresence of the noise created within the measurement system itself.

SUMMARY OF THE INVENTION

The present invention relates to an optical channel monitor thatincorporates optical switching functionality to allow for controlledmonitoring of a set of separate optical fibers (ports) in a WDM system.

In accordance with the present invention, a multiport optical switch(such as an N×1 switch) is used to controllably select a specificincoming optical signal that is to be processed by an included opticalchannel monitor (OCM). The OCM includes a tunable optical filter andphotodetector arrangement, and is configured to measure the opticalspectrum of the incoming optical signal and extract informationassociated with the various optical channels (wavelengths) forming theincoming optical signal (i.e., power, wavelength, OSNR and the like foreach channel). The OCM includes a processor component to perform thisinformation extraction, and also generates a pair of output controlsignals, a first signal to control the wavelength scanning process ofthe tunable optical filter and a second signal to control the setting ofthe multiport optical switch.

In one embodiment of the present invention, the processor componentcontrols the optical switch to optically block all input ports from theoutput port (i.e., creating a “dark” channel input to the OCM). The darkchannel input is used to measure a real-time level of offsets and noisein the OCM electronics and thus provide a baseline noise factor that canbe used to correct subsequent monitoring operations and provide accuratemeasurements of the power of each channel, particularly useful in lowpower conditions.

In yet another embodiment of the present invention, a separatewavelength reference source is included at a selected optical switchinput port and is used to perform self-recalibration of the OCM, asnecessary, to overcome wavelength drift that may have occurred withinthe tunable filter.

In particular, the utilization of the single processor component tocontrol the operation of both the multiport optical switch and thetunable filter allows for several different maintenance/calibrationoperations to be performed. For example, besides wavelength driftcorrection, the processor component can be used to monitor the outputpower level from the tunable filter and provide realignment controlsignals to the tunable filter. Similarly, the alignment between theinput and output ports of the multiport switch can be monitored and afeedback (control) signal used to re-orient one or more of the signalpaths as necessary to provide optimum coupling between a selected inputport and the output port.

In one particular embodiment, the present invention takes the form ofoptical channel monitoring system including the following elements: (1)a multiport optical switch including a plurality of input ports and asingle output port, each input port receiving an optical input signal(each optical input signal including one or more individual wavelengthchannels), with the multiport optical switch controlled to selectivelycouple one input port from the plurality of input ports to the outputport; (2) a tunable optical filter coupled to the output port of themultiport optical switch and responsive to the optical input signal, thetunable optical filter configured to selectively pass separatewavelength channels at different points in time; (3) an opticalphotodetector coupled to the output of the tunable optical filter forconverting each separate wavelength channel into an electrical signalequivalent; and (4) a processor component responsive to the electricalsignal for extracting optical characteristic data used to monitor theperformance of the optical signal. The processor component is furtherconfigured to generate a first control signal applied as an input to themultipart switch to control the input port selection, and a secondcontrol signal applied as an input to the tunable optical filter tocontrol the wavelength channel selection.

Yet another embodiment of the present invention relates to a method ofcontrolling an optical channel monitoring process in a multiportenvironment, the method including the steps of: a) providing a multiportoptical switch at an input of an optical channel monitor, the multiportoptical switch including a plurality of input ports for receiving aplurality of different optical signals and a single output port; b)controlling the multiport optical switch to couple a selected input portto the output port; c) applying the selected optical signal to the inputof the optical channel monitor; d) selecting a center wavelength of atunable optical filter within the optical channel monitor; e) measuringoptical power at the selected center wavelength within the selectedoptical signal; f) repeating steps d) and e) to measure optical power ata set of wavelength channels within the selected optical signal; g)controlling the multiport optical switch to couple a different inputport to the output port; and h) repeating steps b)-g) for one or moreinput ports of the multiport optical switch.

Other and further embodiments and features of the present invention willbecome apparent during the course of the following discussion and byreference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

Referring now to the drawings, where like numerals represent like partsin several views:

FIG. 1 illustrates an exemplary optical channel monitor (OCM)incorporating an optical switch in accordance with the presentinvention;

FIG. 2 is a flowchart illustrating an exemplary set of process stepsthat may be utilized to perform optical channel monitoring with thearrangement of FIG. 1;

FIG. 3 contains plots of per-channel noise error (plot (a)) and totalpower noise error (plot (b)) for a “low gain” configuration;

FIG. 4 contains plots of per-channel noise error (plot (a)) and totalpower noise error (plot (b)) for a “high gain” configuration; and

FIG. 5 illustrates an alternative OCM formed in accordance with thepresent invention, in this case also including a wavelength referencesource.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary optical channel monitoring system 10formed in accordance with the present invention, which incorporates amultiport optical switch 12 at the input of an optical channel monitor(OCM) 14, enabling efficient performance of the OCM in more complexoptical networks, such as those supporting DWDM and using multipleoptical signal paths. OCM 14 is shown as comprising a first module 16that functions to receive an incoming optical signal and provide anelectrical output signal representative of the optical signal. Theincoming optical signal is considered to be supporting the propagationof multiple optical channels, each operating at a different wavelength.First module 16 includes a tunable optical filter 18 that receives theincoming optical signal and scans across a predetermined wavelengthrange of interest for the set of channels associated with that signal(or, perhaps, is adjustably centered on specific wavelength(s)associated with a presented optical signal—perhaps ‘dwelling’ on aparticular wavelength if there are notable problems/errors associatedwith that channel).

The optical output signal from tunable filter 18 is then applied as aninput to a photodiode 20 (or another suitable type of photodetectiondevice) to transform the optical signal into an equivalent electricalrepresentation. The electrical signal takes the form of a raw dataspectrum, which is then applied as an electrical signal input to asignal processing module 22 within a processor component 24. Inaccordance with the conventional operation of an optical channelmonitor, signal processing module 22 is used to analyze this raw dataspectrum and ascertain selected characteristics of the associatedoptical signal (e.g., power, level, wavelength OSNR, etc. of eachseparate channel contained within the incoming optical signal). OCM 14then provides this information as an output information signal to a hostmodule, typically a network management system (NMS), for use incontrolling/correcting the characteristics of each individual channel.

The system of the present invention enhances the operation of aconventional OCM by incorporating multiport optical switch 12 at theinput of OCM 14 and utilizing processor component 24 to control theoperation of both switch 12 and OCM 14 in a manner that allows for animproved efficiency in the monitoring process.

There are a variety of different configurations of a multiport opticalswitch, and various ones of these configurations may be used in thearrangement of the present invention. In general, optical switch 12takes the form of an N×1 optical switch, with a plurality of N inputsignal ports 26, for receiving a plurality of N optical signalspropagating within the communication system, and a single output port28. Optical switch 12 is controlled to couple a selected one of inputports 26 to output port 28. The optical signal exiting at output port 28is then presented as the optical signal to be monitored at the input toOCM 14. OCM 14 then functions in a conventional manner as describedabove to perform optical spectrum measurements of each channel withinfirst module 16 and extract pertinent information regarding theperformance of each channel from this spectral data within signalprocessing module 22 of processor component 24.

As shown in FIG. 1, processor component 24 of OCM 14 also includes acontrol unit 30. In accordance with the present invention, control unit30 is in communication with signal processing module 22 and is used togenerate two output control signals, one for controlling the operationof tunable optical filter 18 and another for controlling the operationof multipart optical switch 12. In a preferred embodiment of the presentinvention, control unit 30 is configured such that these two outputcontrol signals operate in a synchronous manner such that both switch 12and filter 18 change state at the same time.

Referring to FIG. 1, control unit 30 is shown as generating a “channelselect” (or “channel scan”) control signal S₁ that is applied as acontrol input to tunable optical filter 18, and a “port select” controlsignal S₂ that is applied as a control input to multiport optical switch12. Control signal S₁ directs the specific operation of tunable opticalfilter 18 to scan through a particular set of wavelength channelscontained within the optical signal appearing at the input to filter 18.As the filter controls the various wavelength components passingthrough, photodiode 20 then converts the received signal power at eachchannel into an electronic signal for further analysis within signalprocessing module 22 (i.e., measurements of center wavelength, signalpower, OSNR, etc.). This information may be stored within a databaseincluded within processor component 24 (not shown) and/or sent to a hostunit (such as a network monitoring system (NMS)) for additional study.

Once a monitoring operation of a given port is completed, control unit30 sends a “port select” signal S₂ to optical switch 12, instructingswitch 12 select another input port for monitoring. In particular, the“port select” signal S₂ instructs multipart optical switch 12 to bere-configured such that another input port is coupled to output port 28of optical switch 12. In accordance with the present invention, controlunit 30 is configured to also transmit a “channel select” control signalS₁ to tunable filter 18 upon the selection of a new input port atoptical switch 12, instructing filter 18 to initiate a new scan/selectfor a set of wavelengths associated with the channels within the ‘new’signal now appearing at its input. In a preferred embodiment, controlsignals S₁ and S₂ operate in a synchronous manner to reduce latency andimprove the operation efficiency in the multipart OCM environment.

Advantageously, the use of a single component (i.e., processor component24) to control the operation of both optical switch 12 and OCM 14 allowsfor the sharing of computing resources for these two functions, whichwould otherwise require their own processing functionalities, addingsize, complexity and expense to the overall monitoring system. Indeed,by utilizing the monitoring analysis performed by signal processingmodule 22 of OCM 14, control unit 30 can control the operation ofoptical switch 12 so as to “dwell” on a specific port that may beexperiencing problems and allow for continuous measurements to be madefor this port over a longer period of time. Alternatively, control unit30 can control the operation of optical switch 12 so as to monitor someports more regularly than other ports, based on information receivedfrom (for example) the NMS.

In a preferred embodiment of the present invention, signals S₁ and S₂are synchronized so that optical switch 12 moves from one selected portto another in a manner that is synchronous with the re-setting of thecenter wavelength of tunable filter 18. In this case, the latencybetween these other unsynchronized events is minimized, while alsomaximizing the multi-port monitoring capability of OCM 14.

As mentioned above, the utilization of a single processor component inconjunction with both an OCM and multiport switch provides a feedbackarrangement that allows for the operational characteristics of themultiport switch and tunable optical filter to be monitored andre-calibrated and/or adjusted as necessary. For example, as will bediscussed in detail below, wavelength drift within the tunable filtercan be recognized and the control signal input used to re-set thewavelength to the proper, nominal value. The same system can be used tooptimize the output power from the tunable filter by ensuring thatoptical alignment between the input and output signal paths ismaintained. Similarly, it is also possible to utilize the processorcomponent of the present invention to monitor the performance of themultiport switch (in terms of output power efficiency) and utilize thefeedback control signal to realign various switch elements, as necessaryto re-align optical signal paths and provide optimum output power levelfrom the multiport switch.

FIG. 2 is a flowchart illustrating the operation of optical channelmonitoring system 10 of FIG. 1. The process begins at step 100 with aspecific port being selected for monitoring (in this case, designated as“port A” and starting with the value A=1). At step 110, optical switch12 is operated to couple the specific input port 26-A to output port 28,thus providing an input optical signal for monitoring operations as theinput to OCM 14. Optical filtering is then performed on this opticalsignal (shown as step 120 in FIG. 2), where tunable optical filter 18 isscanned across the spectrum of interest (or centered on a predeterminedwavelength/channel) as controlled by channel scan/select control signalS₁. The next operation is shown in step 130 as converting the filteredoptical signal into an electronic representation. An analysis of theelectrical signal is then performed (step 140) to extract informationsuch as power, wavelength and OSNR of the original optical signal. Thisinformation may then be stored in processor component 24 and/or sent toa host module for further analysis (step 150), as well as calculatingany required optimizations of the electrical control signal for theoptical switch and the optical filter.

The process continues at step 160 by activating control unit 30 tocreate “increment port selection” signal. In particular, processcontinues with step 170 sending a “port select” control signal S₂ tooptical switch 12 (shown as “increment port selection”) and step 180sending a “channel scan/select” control signal S₁ to tunable opticalfilter 18. The monitoring process then returns to step 110.

Optical channel monitoring system 10 of FIG. 1 is further capable ofimproving the accuracy of OCM 14 by using a “dark” input to OCM 14 toascertain a background electronic offset and noise level and thereafterremove this baseline noise component from measured optical signals. Thatis, prior to performing monitoring of any of the channels appearing atinput ports 26 of optical switch 12, control unit 30 sends an “off”control signal S₂ to optical switch 12. In response to this signal,optical switch 12 optically de-couples all input ports 26 from outputport 28. As a result, no optical signal is applied as an input to OCM14. Control unit 30 may also function to turn “off” tunable filter 18,thus bypassing the optical filtering properties of OCM 14 as well. Inthis condition, a “no-light” baseline value can be established by signalprocessing module 22 of OCM 14. Thereafter, this baseline amount ofsystem noise is removed from measured optical signal values, improvingthe measurement accuracy of the optical signals, particularly low poweroptical signals.

The ability to perform this baseline measurement eliminates the need tointroduce a front-end DC level, ensuring better low power performance.Additionally, this “dark” measurement also allows for system 10 to beself-recalibrated as needed at any point in time (due to changes inenvironmental conditions, component aging, etc.). That is, control unit30 of processor component 24 can be configured to send the “no-light”control signals to both optical switch 12 and tunable filter 18 on aregular basis to perform an updated measurement of the baseline noisemeasurement, providing an efficient means of maintaining accurateresults in the monitoring process.

FIG. 3 contains a pair of plots illustrating the improvement inperformance when utilizing a noise correction factor, in this exampleassociated with “low gain” conditions (channel power in the range of −20to −40 dBm, and ASE power in range of −40 to −70 dBm). FIG. 3(a) is aplot of error present in channel power measurements, with an indicationof typical specification bars at ±0.6 dB also shown. Both anon-corrected power error measurement dataset (shown as the darker setof points) and a corrected power error measurement dataset (lighter setof points) are plotted in FIG. 3(a). While both the non-corrected andcorrected values are within the industry limits, it is clear that as thechannel power decreases, the non-corrected values tend to spread out,while the corrected dataset values remain clustered near the zero value.

The difference between non-corrected and corrected power measurements isclearer in the total power measurements plotted in FIG. 3(b). Withoutproviding any type of correction, it is evident that the non-correctedpower measurements exhibit a quickly increasing amount of error,particularly as the total power goes below −30 dBm.

FIGS. 4(a) and (b) contain plots similar to those of FIGS. 3(a) and (b),but in this case illustrate data associated with a “high gain” condition(channel power in range of −30 to −50 dBm, same ASE power). Theimprovement obtained by including the noise correction in themeasurements is even more noticeable in this high gain environment.

FIG. 5 illustrates another embodiment of the present invention, in thiscase where a reference wavelength source 40 is included and used tocheck the accuracy of the measurement operations performed by OCM 14. Inparticular, wavelength source 40 provides OCM 14 with an input referencethat can then be used to self-recalibrate over time, compensating forchanges in the operating conditions and/or aging of the optical andelectrical components forming element 16 of OCM 14.

Advantageously, the inclusion of multiport optical switch 12 at theinput to OCM 14 in accordance with the present invention provides asimple access point for coupling reference source 40 into the system. Byutilizing a dedicated input port (shown in this case as port N) onmultipart optical switch 12 as the input for the reference wavelengthsignal, the design simplifies the optical components typically requiredto measure both the conventional input signals and an independentwavelength reference. As mentioned above, the utilization of a singleprocessor component to control the operation of both OCM 14 andmultiport optical switch 12 provides the ability to constantly monitorthe operation of both elements and adjust/re-calibrate their operatingparameters (such as, for example, power optimization). For example,multiport optical switch 12 can be recalibrated by comparing an opticalpower measurement for a given input port setting of the switch to aprevious power measurement for that same port (the previous powermeasurements being stored in a memory element within processor component24. If the current power measurement is too low, control unit 30 willsend a “realignment” control signal to multiport switch 12 thatinitiates an optical realignment process within switch 12 to adjust thesignal path between the given input port and the output port untilmaximum optical coupling is achieved. A similar power measurementprocess may be used between the input and output of tunable opticalfilter 18 to maximum optical coupling through the filter.

While this invention was been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the invention asencompassed by the claims appended hereto.

What is claimed is:
 1. An optical channel monitoring system comprising:a multiport optical switch including a plurality of input ports and asingle output port, each input port receiving a separate optical inputsignal, with each separate optical input signal including a plurality ofseparate wavelength channels, the multiport optical switch controlled toselectively couple one input port from the plurality of input ports tothe single output port; a tunable optical filter coupled to the singleoutput port of the multiport optical switch for receiving the selectedoptical input signal, the tunable optical filter controlled toselectively pass different wavelength channels at different points intime; an optical photodetector coupled to the output of the tunableoptical filter for converting each wavelength channel into an electricalsignal equivalent; and a processor component responsive to theelectrical signal equivalent for extracting optical characteristic datathereform for monitoring the performance of the selected optical signalapplied as an input to the tunable optical filter, the processorcomponent further configured to generate a first control signal appliedas an input to the multiport switch to control the input port selection,and a second control signal applied as an input to the tunable opticalfilter to control the wavelength channel selection, and wherein theprocessor component is further configured to transmit a “disconnect”control signal to the multiport optical switch so as to remove alloptical signal component inputs to the tunable filter and create abaseline noise measurement for use as a correction factor incharacterization of further optical measurements.
 2. The optical channelmonitoring system as defined in claim 1 wherein the processor componentfurther comprises a control unit for generating the first and secondcontrol signals.
 3. The optical channel monitoring system as defined inclaim 2 wherein the signal processor control unit generates a “channelscan/select” control signal to be applied to the tunable optical filterfor controlling the sequence of wavelength components passing throughthe tunable optical filter.
 4. The optical channel monitoring system asdefined in claim 3 wherein the control unit is configured tosimultaneously control the multiport optical switch and the tunableoptical filter.
 5. The optical channel monitoring system as defined inclaim 1 wherein the processor component is configured to transmit a“disconnect” control signal to the tunable optical filter.
 6. Theoptical channel monitoring system as defined in claim 1 wherein thesystem further comprises a reference wavelength source coupled to aninput port of the multiport optical switch, the reference wavelengthsource activated by a separate control signal from the processorcomponent, wherein upon being activated the reference wavelength sourceprovides an optical signal at a known wavelength value as an input tothe system, allowing for the system to perform a self-recalibration tocorrect for any discrepancies between the known wavelength value and ameasured value received at the signal processor.
 7. The optical channelmonitoring system as defined in claim 1 wherein the processor componentis configured to monitor received optical power and utilize the controlsignal input to the tunable optical filter to adjust optical alignmentwithin the tunable optical filter and optimize optical output therefrom.8. The optical channel monitoring system as defined in claim 1 whereinthe processor component is configured to monitor received optical powerand utilize the control signal input to the multiport optical switch toadjust optical alignment within the multiport optical switch andoptimize optical output power therefrom.
 9. A method of controlling anoptical channel monitoring process in a multiport environment, themethod including the steps of: a) providing a multiport optical switchat an input of an optical channel monitor, the multiport optical switchincluding a plurality of input ports for receiving a plurality ofdifferent optical signals and a single output port, and the opticalchannel monitor including a tunable optical filter and a processor forproviding a port selection control signal and a wavelength selectioncontrol signal; b) transmitting a port selection control signal from theprocessor to the multiport optical switch to couple a selected inputport to the output port; c) applying the selected optical signal to theinput of the optical channel monitor; d) transmitting a wavelengthselection control signal from the processor to the tunable opticalfilter to select a specific center wavelength channel for measurementwithin the optical channel monitor; e) measuring optical power at theselected center wavelength within the selected optical signal; f)utilizing the wavelength selection control signal from the processor,repeating steps d) and e) to measure optical power at a set ofwavelength channels within the selected optical signal; g) utilizing theport selection control signal from the processor, operating themultiport optical switch to couple a different input port to the outputport; and h) repeating steps b)-g) for one or more input ports of themultiport optical switch; i) disconnecting all input ports from theoutput port in response to a disconnect port selection control signalfrom the processor; and j) measuring optical power at the output of thetunable optical filter in the absence of any input optical signal; k)defining the optical power measured in step j) as a baseline noisesignal; and l) subtracting the baseline noise signal from measurementsmade in step e).
 10. The method as defined in claim 9 wherein the methodfurther includes a self-recalibration process and comprises theadditional steps of: applying an optical reference signal exhibiting aknown wavelength at an input port of the multiport optical switch;measuring optical power at the output of the tunable optical filter;comparing to known optical reference signal; and recalibrating theoptical channel monitor to correct for differences between the measuredvalues and known reference values.
 11. The method as defined in claim 9wherein the method further includes an optical power self-recalibrationprocess and comprises the additional steps of: measuring optical powerat the output of the tunable optical filter for a given input portsetting of the multiport optical switch; comparing the optical power toprevious measurements at the same optical input port setting; andrecalibrating the multiport optical switch portion of the opticalchannel monitor to maximize the measured optical power for a given inputport by adjusting the optical alignment between the given input port andthe output port of the multiport optical switch.
 12. The method asdefined in claim 9 wherein the method further includes an optical powerself-recalibration process and comprises the additional steps of:measuring optical power at the output of the tunable optical filter fora given input port setting of the multiport optical switch; comparingthe optical power to previous measurements at the same optical inputport setting; and recalibrating the tunable optical filter portion ofthe optical channel monitor to maximize the measured optical power byadjusting the optical alignment within the filter to maximum opticalcoupling efficiency between the filter input and the filter output. 13.The method as defined in claim 9 wherein in performing step d), aselected center wavelength is measured for an extended period of time,dwelling on a selected center wavelength.
 14. The method as defined inclaim 9 wherein in performing step d), a selected center wavelength isscanned across a wavelength range.